NIR-Fluorescent Hybrid Materials of Tm3+ Complexes Carried by Nano-SiO2 via Improved Sol–Gel Method

Tm3+ has obvious emission characteristics in the near-infrared band. Thulium ions combined with different organic ligands lead to different fluorescent properties. In the near-infrared region, Tm3+ is a down-conversion fluorescent material that is unstable under high temperature and acidic conditions. Moreover, in those complex environments, the fluorescence from Tm3+ complex is usually degraded. In this work, two kinds of near-infrared fluorescent complexes, Tm(TTA)3phen and Tm(DBM)3phen, were prepared, and the intensity of their fluorescence is compared. The fluorescence intensity at 802 nm is greatly improved compared with Tm(TTA)3phen, and the intensity of the emission at 1235 nm and 1400–1500 nm is also enhanced. Moreover, the emission lifetime of SiO2-Tm(TTA)3phen is 50.38 μs. Tm(TTA)3phen complex and SiO2-Tm(TTA)3phen hybrid materials have better fluorescence than Tm(DBM)3phen and SiO2-Tm(DBM)3phen. Therefore, HTTA is a better choice of organic ligands for Tm3+. The NIR-fluorescent hybrid materials prepared have stronger fluorescence after combining with nano-SiO2compared with pure Tm3+ complexes, and have stronger structural stability compared with pure nano-SiO2.


Introduction
Rare earth ions complexes have special physical and chemical properties, which have attracted much attention thanks to their high purity, narrow emissivity, and high internal quantum efficiency. The fluorescent properties and mechanism of rare earth ion complexes have been studied for a long time. As early as 1942, Weissman investigated the fluorescence effect of β-diketone ligands on europium ions by ultraviolet radiation, and further studied the fluorescence efficiency of the complex [1][2][3][4]. At present, many organic ligands with large absorption cross sections in ultraviolet region have been found. The energy of the excited state can be transferred to the rare earth ions through the energy transfer inside these organic ligands, so the fluorescence intensity of rare earth ions can be greatly improved. Therefore, the rare earth ions fluorescent complexes formed by the combination of rare earth and organic ligand have excellent fluorescence [5][6][7]. However, the fluorescence properties of rare earth complexes can be damaged in complex environments. In the preparation of many fluorescent materials, rare earth ions cannot be prepared at a high temperature, and the solution concentration should be controlled. Owing to the large amount of coordination with rare earth ions, complexes are easy to coordinate with solution molecules, resulting in fluorescence quenching, and a decrease of the thermal stability. Therefore, high transparency, good rigidity, and three-dimensional network void structure of nano-SiO 2 is chosen as the matrix, which can not only protect the stability of rare earth ions complexes, but also keep or increase the fluorescence of hybrid materials.
Therefore, the hybrid material prepared by the rare earth complex and nano-SiO 2 is a promising strategy to protect the rare earth complex, improve their thermal stability, and to be used to prepare durable functional materials [6]. In order to be used in different fields, there has been a wealth of research on controlling the size of nano-SiO 2 . Rao et al. prepared nano-SiO 2 with the size of 20-460 nm by the simple sol-gel method [8]. Rahman et al. found that, in the presence of a small amount of NH 4 Br, a single size of 20-34 nm could be synthesized [9]. Kim et al. reported that the introduction of dielectric could reduce the size of nano-SiO 2 to 17.5 nm [10]. However, it is difficult to give consideration to both morphology and fluorescence because of the special amorphous structure of nano-SiO 2 , so there are few reports on the rare earth doped silica materials with special morphology [11].
By introducing different templates into the sol-gel method, the micro-morphology of SiO 2 can be controlled. Among them, one template mainly includes surfactants, organic acids, and gel factors [12,13]. Another template mainly includes porous alumina film and insoluble helical polymer. For example, using dodecyl trimethylammonium bromide (CTAB) as a template, spherical and short rod-shaped nano-SiO 2 materials were obtained by adjusting the amount of water. Moreover, different tartaric acid derivatives were used as templates to synthesize nano-SiO 2 hollow microspheres, SiO 2 nanoribbons, and SiO 2 nanotubes [14][15][16]. Compared with previous reports, in order to reduce the influence of H 2 O on the fluorescence of Tm(TTA/DBM) 3 phen, we have improved the experimental method for preparing nano-SiO 2 , and changed the size of nano-SiO 2 by adjusting the dosage of NH 3 ·H 2 O.
Here, we report the synthesis of SiO 2 -Tm 3+ (SiO 2 -Tm(TTA) 3 phen and SiO 2 -Tm(DBM) 3 phen) hybrid materials via an improved method, which not only embeds Tm 3+ complexes inside nano-SiO 2 , but also embeds a part of Tm 3+ complexes on the surface of nano-SiO 2 . Thus, Tm 3+ complexes (Tm(TTA) 3 phen, Tm(DBM) 3 phen) can be more richly filled in SiO 2 , and the fluorescence and stability of Tm 3+ complexes can be enhanced.
Like ordinary electromagnetic waves, infrared light has wave-particle duality. The energy of infrared light is exactly equal to the energy difference between different energy states of molecules, so infrared absorption effect occurs. Because of the differences in fat content, sugar content, and freshness of food, as well as the differences in biological tissues, there are also differences in the absorption of infrared light waves in food and biological tissues. According to the differences in absorption, different food and biological tissues can be detected. Therefore, infrared fluorescent materials with good fluorescence properties provide favorable conditions for the development of biomedical and food detection.

Synthesis of Tm 3+ Complex
The Tm 3+ complex (Tm(TTA) 3 phen, Tm(DBM) 3 phen)was prepared according to the following process. HTTA/HDBM and Phen in a stoichiometric molar ratio were dissolved in a suitable volume of anhydrous ethanol. The mass of HTTA/HDBM and Phen was 1.332 g/1.447 g and 0.396 g respectively, which were mixed, heated, and stirred for 1 h in a beaker. Then, an appropriate amount of 20 µL sodium hydroxide solution was added to the solution. A stoichiometric amount of TmCl3 ethanol solution was then added dropwise to the solution while stirring. The stirring temperature was 60 • C. The molar ratio of Tm 3+ /HTTA/HDBM/Phen was 1:3:1. Then, the pure complexes were collected by centrifugation and alcohol washing for three times. The final products were dried in oven at 50 • C and white powder was obtained.

Synthesis of SiO 2 -Tm 3+ Hybrid Materials
An improved sol-gel process was employed in this work. Firstly, HTTA/HDBM and Phen were dissolved in a certain volume of anhydrous ethanol according to the stoichiometric molar ratio. The mass of HTTA/HDBM and Phen was 1.332 g/1.447 g and 0.396 g, respectively, and the solution was magnetically stirred for 0.5 h at 60 • C at 700 r/min. Next, 3 mL of NH 3 ·H 2 O was added to 50 mL of ethanol. After 30 min, 3 mL TEOS was added to the above solution, then we adjusted the dosage of NH 3 ·H 2 O, ethanol, and TEOS. Compared with previous research, no more water was added. The preparing method was improved and it saved water resources. Then, the proper amounts of silica gel solution, Tm(TTA/DBM) 3 phen solution, and Tm(TTA/DBM) 3 phen powder were mixed, heated, and stirred for 3 h, and then centrifuged (speed: 10,000 r/min, 10 min). The excessive Tm(TTA) 3 phen was removed by three times extensive washing with ethanol. The final products were dried in oven at 50 • C and white powder was obtained.

Results and Discussions
The structure and composition of Tm(TTA) 3 phen were investigated by EDS (Energy dispersive spectrometer) analysis. Figure 1a shows the predicted structure of Tm(TTA) 3 phen and the EDS clearly shows that S, F, N, O, C, and thulium elements are present in the Tm(TTA) 3 phen. According to the element composition obtained by EDS, F and S elements belong to HTTA, while N elements belong to Phen. By calculating the elements content of F, S, N, and thulium, we obtain the ratio between different groups to be Tm 3+ /HTTA/Phen = 1:3:1 [10]. The structure and composition of Tm(DBM) 3 phen are similar to Tm(TTA) 3 phen. N elements belong to Phen and O elements belong to HDBM. By calculating the elements content of C, N, O, and thulium, we obtain the ratio Tm 3+ /DBM/Phen = 1:3:1 [17].  Figure 2 shows the XRD of SiO2, Tm(TTA/DBM)3phen, and SiO2-Tm(TTA/DBM)3phen. Both SiO2-Tm(TTA)3phen and SiO2-Tm(DBM)3phen have a wide diffraction peak at 2θ = 20~25 in Figure  2a,b, which can be assigned to the amorphous silica, and the diffraction peak coincides with the characteristic peak of PDF#47-0715, without significant impurities. It is found that the diffracted intensity of SiO2 in SiO2-Tm(TTA/DBM)3phen hybrid materials is significantly higher than that of pure nano-silica. This is because Tm(TTA/DBM)3phen is embedded in the nano-SiO2, so the bond angle and bond length of SiO2 are changed, which improves the structural symmetry of nano-SiO2. Therefore, the hybrid materials have better structural stability. However, the diffraction peak of nano-SiO2 in SiO2-Tm(TTA)3phen is obviously stronger than that in SiO2-Tm(DBM)3phen, thus the structural stability of SiO2-Tm(TTA)3phen nano hybrid materials is stronger than that of SiO2-Tm(DBM)3phen. Furthermore, in Figure 2a, at the diffraction angle of 2θ = 21.731 and 2θ = 28.337, both of Tm(TTA)3phen and SiO2-Tm(TTA)3phen show a sharp and strong crystallization peak, which indicates that both of Tm(TTA)3phen and SiO2-Tm(TTA)3phen nanoparticles have high crystallinity. Figure 2b indicates that both Tm(DBM)3phen and SiO2-Tm(DBM)3phen nanoparticles have low crystallinity. Therefore, in terms of structure, the structural stability and composite degree of SiO2-Tm(TTA)3phen are obviously stronger than that of SiO2-Tm(DBM)3phen. The structure and morphology of the samples were examined by TEM. From Figure 3a, we can see that the nano hybrid materials prepared by the improved method have good uniformity and dispersion in shape and size. The surface of nano-SiO2 is smooth and the nanospheres have a   Figure 2a,b, which can be assigned to the amorphous silica, and the diffraction peak coincides with the characteristic peak of PDF#47-0715, without significant impurities. It is found that the diffracted intensity of SiO 2 in SiO 2 -Tm(TTA/DBM) 3 phen hybrid materials is significantly higher than that of pure nano-silica. This is because Tm(TTA/DBM) 3 phen is embedded in the nano-SiO 2 , so the bond angle and bond length of SiO 2 are changed, which improves the structural symmetry of nano-SiO 2 . Therefore, the hybrid materials have better structural stability. However, the diffraction peak of nano-SiO 2 in SiO 2 -Tm(TTA) 3 phen is obviously stronger than that in SiO 2 -Tm(DBM) 3 phen, thus the structural stability of SiO 2 -Tm(TTA) 3 phen nano hybrid materials is stronger than that of SiO 2 -Tm(DBM) 3 phen. Furthermore, in Figure 2a, at the diffraction angle of 2θ = 21.731 and 2θ = 28.337, both of Tm(TTA) 3 phen and SiO 2 -Tm(TTA) 3 phen show a sharp and strong crystallization peak, which indicates that both of Tm(TTA) 3 phen and SiO 2 -Tm(TTA) 3 phen nanoparticles have high crystallinity. Figure 2b indicates that both Tm(DBM) 3 phen and SiO 2 -Tm(DBM) 3 phen nanoparticles have low crystallinity. Therefore, in terms of structure, the structural stability and composite degree of SiO 2 -Tm(TTA) 3 phen are obviously stronger than that of SiO 2 -Tm(DBM) 3 phen.   Figure  2a,b, which can be assigned to the amorphous silica, and the diffraction peak coincides with the characteristic peak of PDF#47-0715, without significant impurities. It is found that the diffracted intensity of SiO2 in SiO2-Tm(TTA/DBM)3phen hybrid materials is significantly higher than that of pure nano-silica. This is because Tm(TTA/DBM)3phen is embedded in the nano-SiO2, so the bond angle and bond length of SiO2 are changed, which improves the structural symmetry of nano-SiO2. Therefore, the hybrid materials have better structural stability. However, the diffraction peak of nano-SiO2 in SiO2-Tm(TTA)3phen is obviously stronger than that in SiO2-Tm(DBM)3phen, thus the structural stability of SiO2-Tm(TTA)3phen nano hybrid materials is stronger than that of SiO2-Tm(DBM)3phen. Furthermore, in Figure 2a, at the diffraction angle of 2θ = 21.731 and 2θ = 28.337, both of Tm(TTA)3phen and SiO2-Tm(TTA)3phen show a sharp and strong crystallization peak, which indicates that both of Tm(TTA)3phen and SiO2-Tm(TTA)3phen nanoparticles have high crystallinity. Figure 2b indicates that both Tm(DBM)3phen and SiO2-Tm(DBM)3phen nanoparticles have low crystallinity. Therefore, in terms of structure, the structural stability and composite degree of SiO2-Tm(TTA)3phen are obviously stronger than that of SiO2-Tm(DBM)3phen. The structure and morphology of the samples were examined by TEM. From Figure 3a, we can see that the nano hybrid materials prepared by the improved method have good uniformity and dispersion in shape and size. The surface of nano-SiO2 is smooth and the nanospheres have a The structure and morphology of the samples were examined by TEM. From Figure 3a, we can see that the nano hybrid materials prepared by the improved method have good uniformity and dispersion in shape and size. The surface of nano-SiO 2 is smooth and the nanospheres have a diameter of 80 nm. It is clear from Figure 3b-e that Tm 3+ complexes were successfully attached to the surface of nano-SiO 2 and some of Tm 3+ complexes were embedded into SiO 2 . Figure    According to TEM and XRD characterization, it can be concluded that SiO2 in SiO2-Tm(TTA)3phen hybrid materials provides better protection for Tm(TTA)3phen;combined with the TGA (Thermo Gravimetric Analysis) and fluorescence spectra, it can be concluded that the SiO2-Tm(TTA)3phen has better structural stability and better fluorescent performance than SiO2-Tm(DBM)3phen. Figure 5 shows the FT-IR spectra of the SiO2-Tm(TTA/DBM)3phen hybrid materials, Tm(TTA/DBM)3phen complexes, and nano-SiO2. Figure 5a displays the FT-IR spectra of the Tm(TTA)3phen and SiO2-Tm 3+ hybrid nanoparticles and SiO2. Obviously, the absorption peak of 1061 cm −1 is due to the frequency band of Si-O-Si symmetric telescopic vibration, and 491 cm −1 corresponds to the band of bending vibration of Tm-O-Si. The absorption peak at 791 cm −1 is the bending vibration peak of Si-OH. FT-IR spectra indicate that Tm(TTA)3phen and SiO2 combine to a   According to TEM and XRD characterization, it can be concluded that SiO2 in SiO2-Tm(TTA)3phen hybrid materials provides better protection for Tm(TTA)3phen;combined with the TGA (Thermo Gravimetric Analysis) and fluorescence spectra, it can be concluded that the SiO2-Tm(TTA)3phen has better structural stability and better fluorescent performance than SiO2-Tm(DBM)3phen. Figure 5 shows the FT-IR spectra of the SiO2-Tm(TTA/DBM)3phen hybrid materials, Tm(TTA/DBM)3phen complexes, and nano-SiO2. Figure 5a displays the FT-IR spectra of the Tm(TTA)3phen and SiO2-Tm 3+ hybrid nanoparticles and SiO2. Obviously, the absorption peak of 1061 cm −1 is due to the frequency band of Si-O-Si symmetric telescopic vibration, and 491 cm −1 corresponds to the band of bending vibration of Tm-O-Si. The absorption peak at 791 cm −1 is the bending vibration peak of Si-OH. FT-IR spectra indicate that Tm(TTA)3phen and SiO2 combine to a According to TEM and XRD characterization, it can be concluded that SiO 2 in SiO 2 -Tm(TTA) 3 phen hybrid materials provides better protection for Tm(TTA) 3 phen;combined with the TGA (Thermo Gravimetric Analysis) and fluorescence spectra, it can be concluded that the SiO 2 -Tm(TTA) 3 phen has better structural stability and better fluorescent performance than SiO 2 -Tm(DBM) 3 phen. Figure 5 shows the FT-IR spectra of the SiO 2 -Tm(TTA/DBM) 3 phen hybrid materials, Tm(TTA/DBM) 3 phen complexes, and nano-SiO 2 . Figure 5a displays the FT-IR spectra of the Tm(TTA) 3 phen and SiO 2 -Tm 3+ hybrid nanoparticles and SiO 2 . Obviously, the absorption peak of 1061 cm −1 is due to the frequency band of Si-O-Si symmetric telescopic vibration, and 491 cm −1 corresponds to the band of bending vibration of Tm-O-Si. The absorption peak at 791 cm −1 is the bending vibration peak of Si-OH. FT-IR spectra indicate that Tm(TTA) 3 phen and SiO 2 combine to a hybrid material with a stable structure. Figure 5b shows the FT-IR spectra of the HTTA, Phen, and Tm(TTA) 3 phen. For Tm(TTA) 3 phen, the characteristic bands at 1577 cm −1 and 1656 cm −1 of HTTA disappear. At the same time, a strong absorption peak (C=O) appears at 1603 cm −1 , indicating that the carbonyl group in HTTA is coordinated with Tm 3+ , and HTTA is confirmed as a non-negative bidentate ligand to coordinate with Tm 3+ . The coordination bond between Tm 3+ and Phen is also formed in Tm(TTA) 3 phen. The absorption peak (C=N) of Phen appears around 1538 cm −1 . The absorption peaks (N-H) at 3370 cm −1 and 3100 cm −1 in Phen disappear and form new coordination bonds with thulium ions. This indicates that the coordination between the two carbon atoms of Phen and thulium ions is bidentate coordination. FT-IR spectra indicate that Tm(TTA) 3 phen and SiO 2 combine to form a hybrid material with a stable structure [18]. Similarly, when Tm(DBM) 3 phen is successfully combined with SiO 2 in Figure 5c,d, the absorption peaks (N-H) at 3370 cm −1 and 3100 cm −1 in Phen disappear and form new coordination bonds with thulium ions. The stretching vibration of C=N originally located at 1413 cm −1 is red shifted, indicating that C=N is weakened to a certain extent. C=N and nano silicon hydroxyl on the surface of silicon are bonded by hydrogen to provide a site for the Tm 3+ complex, while its strength is weakened. The absorption peak at 791 cm −1 is the bending vibration peak of Si-OH, while the peak at 491 cm −1 corresponds to the band of bending vibration of Tm-O-Si [18].
Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 13 hybrid material with a stable structure. Figure 5b shows the FT-IR spectra of the HTTA, Phen, and Tm(TTA)3phen. For Tm(TTA)3phen, the characteristic bands at 1577 cm −1 and 1656 cm −1 of HTTA disappear. At the same time, a strong absorption peak (C=O) appears at 1603 cm −1 , indicating that the carbonyl group in HTTA is coordinated with Tm 3+ , and HTTA is confirmed as a non-negative bidentate ligand to coordinate with Tm 3+ . The coordination bond between Tm 3+ and Phen is also formed in Tm(TTA)3phen. The absorption peak (C=N) of Phen appears around 1538 cm −1 . The absorption peaks (N-H) at 3370 cm −1 and 3100 cm −1 in Phen disappear and form new coordination bonds with thulium ions. This indicates that the coordination between the two carbon atoms of Phen and thulium ions is bidentate coordination. FT-IR spectra indicate that Tm(TTA)3phen and SiO2 combine to form a hybrid material with a stable structure [18]. Similarly, when Tm(DBM)3phen is successfully combined with SiO2 in Figure 5c Figure 6 shows the TGA curves of SiO2-Tm(DBM)3phen, SiO2-Tm(TTA)3phen, Tm(TTA)3phen, and Tm(DBM)3phen, respectively. By thermo gravimetric analysis of SiO2-Tm(DBM)3phen, SiO2-Tm(TTA)3phen, Tm(TTA)3phen, and Tm(DBM)3phen, the thermal stability of the Tm(TTA/DBM)3phen and the SiO2-Tm(TTA/DBM)3phen was obtained, respectively. As can be seen from Figure 6, the thermal decomposition of Tm(TTA)3phen complexes shows 60% weight loss in the range of 263 °C to 380 °C, which is analyzed as the decomposition of two organic ligands in the Tm(TTA)3phen. After 400 °C, all the complexes decompose to thulium oxide. In addition, the thermal decomposition of SiO2-Tm(TTA)3phen is divided into three stages. In the first stage, there is 3% weight loss in the range of 40 °C to 100 °C, which is due to the removal of the adsorbed water on the surface of SiO2-Tm(TTA)3phen nanosphere. In the second stage, there is 7% weight loss in the   3 phen was obtained, respectively. As can be seen from Figure 6, the thermal decomposition of Tm(TTA) 3 phen complexes shows 60% weight loss in the range of 263 • C to 380 • C, which is analyzed as the decomposition of two organic ligands in the Tm(TTA) 3 phen. After 400 • C, all the complexes decompose to thulium oxide. In addition, the thermal decomposition of SiO 2 -Tm(TTA) 3 phen is divided into three stages. In the first stage, there is 3% weight loss in the range of 40 • C to 100 • C, which is due to the removal of the adsorbed water on the surface of SiO 2 -Tm(TTA) 3 phen nanosphere. In the second stage, there is 7% weight loss in the range of 110 • C to 330 • C, which is due to the dehydration of water of colloidal nano-SiO 2 in SiO 2 -Tm(TTA) 3 phen molecule. In the third stage, there is 3% weight loss in the range of 330 • C to 500 • C, which is due to the dehydration of crystal water in SiO 2 -Tm(TTA) 3 phen molecules. Thus, it can be seen that the thermal stability of Tm(TTA) 3 phen is greatly improved after combination with nano-SiO 2 . Moreover, by analyzing the thermal weight of SiO 2 -Tm(DBM) 3 phen and Tm(DBM) 3 phen, we can see that the initial decomposition temperature of the two nanomaterials is 220 • C and 300 • C, respectively. Therefore, the thermal stability of SiO 2 -Tm(DBM) 3 phen and Tm(DBM) 3 phen is less stable than SiO 2 -Tm(TTA) 3 phen; this is consistent with the results of TEM images analysis. Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 13 range of 110 °C to 330 °C, which is due to the dehydration of water of colloidal nano-SiO2 in SiO2-Tm(TTA)3phen molecule. In the third stage, there is 3% weight loss in the range of 330 °C to 500 °C, which is due to the dehydration of crystal water in SiO2-Tm(TTA)3phen molecules. Thus, it can be seen that the thermal stability of Tm(TTA)3phen is greatly improved after combination with nano-SiO2. Moreover, by analyzing the thermal weight of SiO2-Tm(DBM)3phen and Tm(DBM)3phen, we can see that the initial decomposition temperature of the two nanomaterials is 220 °C and 300 °C, respectively. Therefore, the thermal stability of SiO2-Tm(DBM)3phen and Tm(DBM)3phen is less stable than SiO2-Tm(TTA)3phen; this is consistent with the results of TEM images analysis.  Figure 7 shows the Jablonski diagram of prepared samples. The intra-molecular energy transfer mechanism shown in Figure 7a is the energy transfer process in Tm 3+ complexes, which most scientists agree with Crosby's theory [19] on the energy transfer mechanism of organic ligands transferring to Tm 3+ . In this process, Tm 3+ is no longer first excited by the external excitation energy, but the organicligands of the Tm 3+ complex first absorb energy, which is excited from the ground state S0 to the first excited state S1. Then, the molecules are internally transformed to the low-excited single state S1. In the next step of the process, the energy is transferred from the lowest excited singlet S1 to the excited triplet T1. Next, the energy is transferred from the lowest excited triplet T1 to the Tm 3+ by virtue of the chemical bond of the Tm 3+ complex itself. Thulium ion receives energy and is stimulated to produce an energy level transition, emitting characteristic fluorescence as they return to the ground state. If the energy level of the lowest excited triplet (T1) is lower than that of the excited state of rare earth ion, effective energy transfer cannot occur [20]. Recursively, the effect of this organic ligand′s energy transfer to the fluorescence material central ion is called the "antenna effect" [21,22]. Because of the stronger absorption capacity of HTTA in ultraviolet region and higher excitation state energy transfer efficiency than HDBM, HTTA helps to shorten the transition state of thulium ion and enhance the absorption coefficient of thulium ion in ultraviolet region. As the absorption coefficient of lanthanide ions is extremely low, and f-f level transitions of rare earth ions are forbidden, the ions must be combined with appropriate ligands. In this case, the excitation energy generated by the ligands absorbing photons in the ultraviolet region is transferred to the 4f resonance level [23]. Basically, intra-molecular energy transfer in thulium ion complex molecule is strongly affected by the energy difference between T1 of donor and 4f electronic levels of thulium ions acceptor [24][25][26]. In the energy transfer process from organic ligands to thulium ion, the optimal energy difference is conducive to effective energy transfer. TheT1 of HTTA and HDBM are 20,400 cm −1 and 21,700 cm −1 , respectively. Moreover, the energy difference between the 3 F2 level of HTTA and thulium ion is 7900 cm −1 [27], which is obviously lower than the energy difference between the  Figure 7 shows the Jablonski diagram of prepared samples. The intra-molecular energy transfer mechanism shown in Figure 7a is the energy transfer process in Tm 3+ complexes, which most scientists agree with Crosby's theory [19] on the energy transfer mechanism of organic ligands transferring to Tm 3+ . In this process, Tm 3+ is no longer first excited by the external excitation energy, but the organicligands of the Tm 3+ complex first absorb energy, which is excited from the ground state S 0 to the first excited state S 1 . Then, the molecules are internally transformed to the low-excited single state S 1 . In the next step of the process, the energy is transferred from the lowest excited singlet S 1 to the excited triplet T 1 . Next, the energy is transferred from the lowest excited triplet T 1 to the Tm 3+ by virtue of the chemical bond of the Tm 3+ complex itself. Thulium ion receives energy and is stimulated to produce an energy level transition, emitting characteristic fluorescence as they return to the ground state. If the energy level of the lowest excited triplet (T 1 ) is lower than that of the excited state of rare earth ion, effective energy transfer cannot occur [20]. Recursively, the effect of this organic ligand s energy transfer to the fluorescence material central ion is called the "antenna effect" [21,22]. Because of the stronger absorption capacity of HTTA in ultraviolet region and higher excitation state energy transfer efficiency than HDBM, HTTA helps to shorten the transition state of thulium ion and enhance the absorption coefficient of thulium ion in ultraviolet region. As the absorption coefficient of lanthanide ions is extremely low, and f-f level transitions of rare earth ions are forbidden, the ions must be combined with appropriate ligands. In this case, the excitation energy generated by the ligands absorbing photons in the ultraviolet region is transferred to the 4f resonance level [23]. Basically, intra-molecular energy transfer in thulium ion complex molecule is strongly affected by the energy difference between T 1 of donor and 4f electronic levels of thulium ions acceptor [24][25][26]. In the energy transfer process from organic ligands to thulium ion, the optimal energy difference is conducive to effective energy transfer. TheT 1 of HTTA and HDBM are 20,400 cm −1 and 21,700 cm −1 , respectively. Moreover, the energy difference between the 3 F 2 level of HTTA and thulium ion is 7900 cm −1 [27], which is obviously lower than the energy difference between the 3 F 2 level of HDBM and thulium ion. Therefore, Tm(TTA) 3 phen will have stronger fluorescence than that of Tm(DBM) 3 phen. Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 13 3 F2 level of HDBM and thulium ion. Therefore, Tm(TTA)3phen will have stronger fluorescence than that of Tm(DBM)3phen. The excitation spectra of Tm 3+ complexes and SiO2-Tm 3+ hybrid materials are shown in Figure  7b. The excitation spectra of two thulium ions complexes containing different organic ligands have wide absorption bands, which are mainly attributed to the π-π* electron transfer of the organic ligands. However, the absorption band of HTTA is significantly stronger than that of HDBM. In addition, the fluorescence sensitization of HTTA is much more effective than that of HDBM. The excitation spectrum of SiO2-Tm(TTA)3phen is narrower and stronger than that of Tm(TTA)3phen, Tm(DBM)3phen, and SiO2-Tm(DBM)3phen, but the location of the excitation peak and the absorption band position of the organic ligands remain unchanged. This suggests that the introduction of SiO2 does not alter the absorption bands of the organic ligands. Moreover, the coordination bond formed by SiO2 and Tm 3+ can carry out more efficient energy transfer in thulium ions complexes molecules. Four samples with the strongest fluorescence spectra were detected at the excitation wavelength of 370 nm. Figure 7c-f show the emission spectra of Tm(TTA/DBM)3phen and SiO2-Tm(TTA/DBM)3phen hybrid materials. In Figure 7c, the emission peak of SiO2-Tm(TTA)3phen is basically unchanged compared with Tm(TTA)3phen, while the fluorescence intensity is obviously enhanced. Tm(TTA)3phen and SiO2-Tm(TTA)3phen exhibit sharp peaks at 799 nm and 802 nm that are ascribed to 3 H4→ 3 H6 transitions of Tm 3+ [28]. A wide emission peak appears at 1400 nm-1550 nm, which is caused by the 3 H4→ 3 F4 transitions of Tm 3+ . The emission peak of SiO2-Tm(TTA)3phen is higher and wider than that of Tm(TTA)3phen [29][30][31]. The highest intensity of Tm(TTA)3phen in the curve is 1387 (a.u.), while the highest intensity of SiO2-Tm(TTA)3phen in the curve is 2630 (a.u.), thus there is an enhancement about two times compared with pure complex; the data are given in Table 1. When HDBM is introduced as the ligand of trivalent thulium ion, the highest intensity of Tm(DBM)3phen in the curve is 1235 (a.u.), while the highest intensity of SiO2-Tm(DBM)3phen in the curve is 1280 (a.u.). Thus, there is also an enhancement compared with pure complex; the data are also shown in Table 1. In summary, the NIR-fluorescent hybrid materials have stronger fluorescence after combination with nano-SiO2 compared with pure Tm 3+ complexes, and SiO2-Tm(TTA)3phen is more The excitation spectra of Tm 3+ complexes and SiO 2 -Tm 3+ hybrid materials are shown in Figure 7b. The excitation spectra of two thulium ions complexes containing different organic ligands have wide absorption bands, which are mainly attributed to the π-π* electron transfer of the organic ligands. However, the absorption band of HTTA is significantly stronger than that of HDBM. In addition, the fluorescence sensitization of HTTA is much more effective than that of HDBM. The excitation spectrum of SiO 2 -Tm(TTA) 3 phen is narrower and stronger than that of Tm(TTA) 3 phen, Tm(DBM) 3 phen, and SiO 2 -Tm(DBM) 3 phen, but the location of the excitation peak and the absorption band position of the organic ligands remain unchanged. This suggests that the introduction of SiO 2 does not alter the absorption bands of the organic ligands. Moreover, the coordination bond formed by SiO 2 and Tm 3+ can carry out more efficient energy transfer in thulium ions complexes molecules. Four samples with the strongest fluorescence spectra were detected at the excitation wavelength of 370 nm. Figure 7c-f show the emission spectra of Tm(TTA/DBM) 3 phen and SiO 2 -Tm(TTA/DBM) 3 phen hybrid materials. In Figure 7c, the emission peak of SiO 2 -Tm(TTA) 3 phen is basically unchanged compared with Tm(TTA) 3 phen, while the fluorescence intensity is obviously enhanced. Tm(TTA) 3 phen and SiO 2 -Tm(TTA) 3 phen exhibit sharp peaks at 799 nm and 802 nm that are ascribed to 3 H 4 → 3 H 6 transitions of Tm 3+ [28]. A wide emission peak appears at 1400 nm-1550 nm, which is caused by the 3 H 4 → 3 F 4 transitions of Tm 3+ . The emission peak of SiO 2 -Tm(TTA) 3 phen is higher and wider than that of Tm(TTA) 3 phen [29][30][31]. The highest intensity of Tm(TTA) 3 phen in the curve is 1387 (a.u.), while the highest intensity of SiO 2 -Tm(TTA) 3 phen in the curve is 2630 (a.u.), thus there is an enhancement about two times compared with pure complex; the data are given in Table 1. When HDBM is introduced as the ligand of trivalent thulium ion, the highest intensity of Tm(DBM) 3 phen in the curve is 1235 (a.u.), while the highest intensity of SiO 2 -Tm(DBM) 3 phen in the curve is 1280 (a.u.). Thus, there is also an enhancement compared with pure complex; the data are also shown in Table 1. In summary, the NIR-fluorescent hybrid materials have stronger fluorescence after combination with nano-SiO 2 compared with pure Tm 3+ complexes, and SiO 2 -Tm(TTA) 3 phen is more fluorescent than SiO 2 -Tm(DBM) 3 phen [32]. In addition, from Figure 7e, we can see the emission peak position of SiO2-Tm(DBM)3phen is basically unchanged compared with Tm(DBM)3phen, but it is different from Figure 7c; that is, the emission intensity of SiO2-Tm(DBM)3phen near 800 nm is slightly higher than that of Tm(DBM)3phen. The emission peak of SiO2-Tm(DBM)3phen at 1243 nm is originally the emission peak of Tm(DBM)3phen at 971nm. The fluorescence intensity of SiO2-Tm(DBM)3phen near 1400-1500 nm is higher than Tm(DBM)3phen. Figure 7d,f show that the fluorescence intensity of both Tm(DBM)3phen and SiO2-Tm(DBM)3phen is much lower than that of Tm(TTA)3phen and SiO2-Tm(TTA)3phen.
The UV-vis absorption spectra of Tm(TTA)3phen and Tm(DBM)3phen are shown in Figure S1. It can be found that the absorption peak of Tm(TTA)3phen and Tm(DBM)3phen is the strongest near 370 nm. According to the ultraviolet absorption spectrum analysis, the strong absorption capacity of HTTA/HDBM, and the high energy transfer efficiency of excited state in the ultraviolet region, HTTA/HDBM can help shorten the transition state of thulium ion in the ultraviolet region and improve the absorption coefficient of thulium ion in the ultraviolet region. The absorptive bandwidth of Tm(TTA)3phen is wider than that of Tm(DBM)3phen, combined with UV-vis absorption spectra shown in Figure S1 of Tm(DBM)3phen and Tm(TTA)3phen, which indicates that HTTA absorbs energy and delivers it more efficiently to Tm 3+ , resulting in better sensitized radiation of Tm 3+ . Therefore, HTTA has a better matching degree with Tm 3+ than HDBM, which also indicates that Tm(TTA)3phen has a higher fluorescence yield than Tm(DBM)3phen.
The increase of fluorescence intensity of SiO2-Tm 3+ hybrid materials is due to the carrying effect of SiO2-Tm 3+ . The amorphous nano-SiO2 provide a microenvironment for Tm 3+ complex to limit energy consumption by reducing non-radiative transitions, leading to an increase in the fluorescence intensity of SiO2-Tm 3+ . According to the analysis of fluorescence spectra (Figure 7) and ultraviolet absorption spectrum( Figure S1), we find that Tm(TTA)3phen and SiO2-Tm(TTA)3phen hybrid nanoparticles have higher fluorescence than those of Tm(DBM)3phen and SiO2-Tm(DBM)3phen. These results show that HTTA is a better choice of organic ligands for Tm 3+ , and SiO2-Tm(TTA)3phen still has better fluorescence after the combination with nano-SiO2.
To further investigate the fluorescence properties of thulium ion complexes, the room-temperature fluorescence decay curves were measured in Figure S2, excited at 370 nm, and monitored at 802 nm. The decay curve of SiO2-Tm(TTA)3phen fits a single exponential function: D(t) = c0exp(−t/τ), with a lifetime of 50.38 μs. Figure 8 shows the TEM images of SiO2-Tm 3+ hybrid fluorescent materials with different particle size prepared under different NH3·H2O conditions. The measurements show that, when the dosage of NH3·H2O is increased from 1 mL to 3 mL, the diameters of SiO2-Tm 3+ nanospheres are gradually increased and the dispersibility of the nanospheres are improved. Moreover, as can be seen in Figure 8, when the diameter of the SiO2-Tm 3+ nanoparticles is 80 nm, the morphology of the hybrid fluorescent materials is uniform and maintains good dispersity. More Tm 3+ complexes are attached to the nano-SiO2 nanoparticles in this case.  In addition, from Figure 7e, we can see the emission peak position of SiO2-Tm(DBM)3phen is basically unchanged compared with Tm(DBM)3phen, but it is different from Figure 7c; that is, the emission intensity of SiO2-Tm(DBM)3phen near 800 nm is slightly higher than that of Tm(DBM)3phen. The emission peak of SiO2-Tm(DBM)3phen at 1243 nm is originally the emission peak of Tm(DBM)3phen at 971nm. The fluorescence intensity of SiO2-Tm(DBM)3phen near 1400-1500 nm is higher than Tm(DBM)3phen. Figure 7d,f show that the fluorescence intensity of both Tm(DBM)3phen and SiO2-Tm(DBM)3phen is much lower than that of Tm(TTA)3phen and SiO2-Tm(TTA)3phen.
The UV-vis absorption spectra of Tm(TTA)3phen and Tm(DBM)3phen are shown in Figure S1. It can be found that the absorption peak of Tm(TTA)3phen and Tm(DBM)3phen is the strongest near 370 nm. According to the ultraviolet absorption spectrum analysis, the strong absorption capacity of HTTA/HDBM, and the high energy transfer efficiency of excited state in the ultraviolet region, HTTA/HDBM can help shorten the transition state of thulium ion in the ultraviolet region and improve the absorption coefficient of thulium ion in the ultraviolet region. The absorptive bandwidth of Tm(TTA)3phen is wider than that of Tm(DBM)3phen, combined with UV-vis absorption spectra shown in Figure S1 of Tm(DBM)3phen and Tm(TTA)3phen, which indicates that HTTA absorbs energy and delivers it more efficiently to Tm 3+ , resulting in better sensitized radiation of Tm 3+ . Therefore, HTTA has a better matching degree with Tm 3+ than HDBM, which also indicates that Tm(TTA)3phen has a higher fluorescence yield than Tm(DBM)3phen.
The increase of fluorescence intensity of SiO2-Tm 3+ hybrid materials is due to the carrying effect of SiO2-Tm 3+ . The amorphous nano-SiO2 provide a microenvironment for Tm 3+ complex to limit energy consumption by reducing non-radiative transitions, leading to an increase in the fluorescence intensity of SiO2-Tm 3+ . According to the analysis of fluorescence spectra( Figure 7) and ultraviolet absorption spectrum( Figure S1), we find that Tm(TTA)3phen and SiO2-Tm(TTA)3phen hybrid nanoparticles have higher fluorescence than those of Tm(DBM)3phen and SiO2-Tm(DBM)3phen. These results show that HTTA is a better choice of organic ligands for Tm 3+ , and SiO2-Tm(TTA)3phen still has better fluorescence after the combination with nano-SiO2.
To further investigate the fluorescence properties of thulium ion complexes, the room-temperature fluorescence decay curves were measured in Figure S2, excited at 370 nm, and monitored at 802 nm. The decay curve of SiO2-Tm(TTA)3phen fits a single exponential function: D(t) = c0exp(−t/τ), with a lifetime of 50.38 μs. Figure 8 shows the TEM images of SiO2-Tm 3+ hybrid fluorescent materials with different particle size prepared under different NH3·H2O conditions. The measurements show that, when the dosage of NH3·H2O is increased from 1 mL to 3 mL, the diameters of SiO2-Tm 3+ nanospheres are gradually increased and the dispersibility of the nanospheres are improved. Moreover, as can be seen in Figure 8, when the diameter of the SiO2-Tm 3+ nanoparticles is 80 nm, the morphology of the hybrid fluorescent materials is uniform and maintains good dispersity. More Tm 3+ complexes are attached to the nano-SiO2 nanoparticles in this case. In addition, from Figure 7e, we can see the emission peak position of SiO 2 -Tm(DBM) 3 phen is basically unchanged compared with Tm(DBM) 3 phen, but it is different from Figure 7c; that is, the emission intensity of SiO 2 -Tm(DBM) 3 phen near 800 nm is slightly higher than that of Tm(DBM) 3 phen. The emission peak of SiO 2 -Tm(DBM) 3 phen at 1243 nm is originally the emission peak of Tm(DBM) 3 phen at 971nm. The fluorescence intensity of SiO 2 -Tm(DBM) 3 phen near 1400-1500 nm is higher than Tm(DBM) 3 phen. Figure 7d,f show that the fluorescence intensity of both Tm(DBM) 3 phen and SiO 2 -Tm(DBM) 3 phen is much lower than that of Tm(TTA) 3 phen and SiO 2 -Tm(TTA) 3 phen.
The UV-vis absorption spectra of Tm(TTA) 3 phen and Tm(DBM) 3 phen are shown in Figure S1. It can be found that the absorption peak of Tm(TTA) 3 phen and Tm(DBM) 3 phen is the strongest near 370 nm. According to the ultraviolet absorption spectrum analysis, the strong absorption capacity of HTTA/HDBM, and the high energy transfer efficiency of excited state in the ultraviolet region, HTTA/HDBM can help shorten the transition state of thulium ion in the ultraviolet region and improve the absorption coefficient of thulium ion in the ultraviolet region. The absorptive bandwidth of Tm(TTA) 3 phen is wider than that of Tm(DBM) 3 phen, combined with UV-vis absorption spectra shown in Figure S1 of Tm(DBM) 3 phen and Tm(TTA) 3 phen, which indicates that HTTA absorbs energy and delivers it more efficiently to Tm 3+ , resulting in better sensitized radiation of Tm 3+ . Therefore, HTTA has a better matching degree with Tm 3+ than HDBM, which also indicates that Tm(TTA) 3 phen has a higher fluorescence yield than Tm(DBM) 3 phen.
The increase of fluorescence intensity of SiO 2 -Tm 3+ hybrid materials is due to the carrying effect of SiO 2 -Tm 3+ . The amorphous nano-SiO 2 provide a microenvironment for Tm 3+ complex to limit energy consumption by reducing non-radiative transitions, leading to an increase in the fluorescence intensity of SiO 2 -Tm 3+ . According to the analysis of fluorescence spectra( Figure 7) and ultraviolet absorption spectrum( Figure S1), we find that Tm(TTA) 3 phen and SiO 2 -Tm(TTA) 3 phen hybrid nanoparticles have higher fluorescence than those of Tm(DBM) 3 phen and SiO 2 -Tm(DBM) 3 phen. These results show that HTTA is a better choice of organic ligands for Tm 3+ , and SiO 2 -Tm(TTA) 3 phen still has better fluorescence after the combination with nano-SiO 2 .
To further investigate the fluorescence properties of thulium ion complexes, the room-temperature fluorescence decay curves were measured in Figure S2, excited at 370 nm, and monitored at 802 nm. The decay curve of SiO 2 -Tm(TTA) 3 phen fits a single exponential function: D(t) = c 0 exp(−t/τ), with a lifetime of 50.38 µs. Figure 8 shows the TEM images of SiO 2 -Tm 3+ hybrid fluorescent materials with different particle size prepared under different NH 3 ·H 2 O conditions. The measurements show that, when the dosage of NH 3 ·H 2 O is increased from 1 mL to 3 mL, the diameters of SiO 2 -Tm 3+ nanospheres are gradually increased and the dispersibility of the nanospheres are improved. Moreover, as can be seen in Figure 8, when the diameter of the SiO 2 -Tm 3+ nanoparticles is 80 nm, the morphology of the hybrid fluorescent materials is uniform and maintains good dispersity. More Tm 3+ complexes are attached to the nano-SiO 2 nanoparticles in this case. The fluorescence spectra of SiO2-Tm 3+ hybrid fluorescent materials (SiO2-Tm(TTA)3phen and SiO2-Tm(DBM)3phen) of different particle sizes are shown in Figure 9. Of the four different sizes of SiO2-Tm(TTA)3phen, when the diameter of nanoparticles is 80 nm, the fluorescence intensity of SiO2-Tm(TTA)3phen is the highest. Similarly, of the four different sizes of SiO2-Tm(DBM)3phen, when the diameter of nanoparticles is 80 nm, the fluorescence intensity of SiO2-Tm(DBM)3phen is the highest. However, the fluorescence intensity of SiO2-Tm 3+ hybrid fluorescent material with a diameter of 40 nm is higher than that of SiO2-Tm 3+ hybrid fluorescent material with a diameter of 50 nm. This is because, when the content of NH3·H2O is high, the hydrolysis process of TEOS is accelerated. As the nano-SiO2 with a diameter of 40 nm has better dispersion, more Tm(TTA)3phen complexes get attached to each nano-SiO2 and embedded in the nanospheres in samples of the same volume. This gives higher fluorescence intensity.  The fluorescence spectra of SiO 2 -Tm 3+ hybrid fluorescent materials (SiO 2 -Tm(TTA) 3 phen and SiO 2 -Tm(DBM) 3 phen) of different particle sizes are shown in Figure 9. Of the four different sizes of SiO 2 -Tm(TTA) 3 phen, when the diameter of nanoparticles is 80 nm, the fluorescence intensity of SiO 2 -Tm(TTA) 3 phen is the highest. Similarly, of the four different sizes of SiO 2 -Tm(DBM) 3 phen, when the diameter of nanoparticles is 80 nm, the fluorescence intensity of SiO 2 -Tm(DBM) 3 phen is the highest. However, the fluorescence intensity of SiO 2 -Tm 3+ hybrid fluorescent material with a diameter of 40 nm is higher than that of SiO 2 -Tm 3+ hybrid fluorescent material with a diameter of 50 nm. This is because, when the content of NH 3 ·H 2 O is high, the hydrolysis process of TEOS is accelerated. As the nano-SiO 2 with a diameter of 40 nm has better dispersion, more Tm(TTA) 3 phen complexes get attached to each nano-SiO 2 and embedded in the nanospheres in samples of the same volume. This gives higher fluorescence intensity. The fluorescence spectra of SiO2-Tm 3+ hybrid fluorescent materials (SiO2-Tm(TTA)3phen and SiO2-Tm(DBM)3phen) of different particle sizes are shown in Figure 9. Of the four different sizes of SiO2-Tm(TTA)3phen, when the diameter of nanoparticles is 80 nm, the fluorescence intensity of SiO2-Tm(TTA)3phen is the highest. Similarly, of the four different sizes of SiO2-Tm(DBM)3phen, when the diameter of nanoparticles is 80 nm, the fluorescence intensity of SiO2-Tm(DBM)3phen is the highest. However, the fluorescence intensity of SiO2-Tm 3+ hybrid fluorescent material with a diameter of 40 nm is higher than that of SiO2-Tm 3+ hybrid fluorescent material with a diameter of 50 nm. This is because, when the content of NH3·H2O is high, the hydrolysis process of TEOS is accelerated. As the nano-SiO2 with a diameter of 40 nm has better dispersion, more Tm(TTA)3phen complexes get attached to each nano-SiO2 and embedded in the nanospheres in samples of the same volume. This gives higher fluorescence intensity.

Conclusions
In summary, SiO 2 -Tm(TTA/DBM) 3 phen NIR-fluorescent hybrid materials were prepared by an improved sol-gel method. By comparing the thulium ion complexes of two different organic ligands, we found that Tm(TTA) 3 phen has better fluorescent performance than that of Tm(DBM) 3 phen. In addition, the hybrid materials ofSiO 2 -Tm(TTA) 3 phen have better structural stability and thermal stability than that of SiO 2 -Tm(DBM) 3 phen. Moreover, fluorescence intensity of SiO 2 -Tm(TTA) 3 phen at 802 nm, 1231 nm, and 1400-1500 nm is significantly higher than that of Tm(TTA) 3 phen, Tm(DBM) 3 phen, and SiO 2 -Tm(DBM) 3 phen. Tm(TTA) 3 phen and SiO 2 -Tm(TTA) 3 phendisplay better fluorescent performance than that of Tm(DBM) 3 phen and SiO 2 -Tm(DBM) 3 phen. Therefore, HTTA is a better choice of organic ligand for Tm 3+ , and SiO 2 -Tm(TTA) 3 phen still has better fluorescent performance after the combination with nano-SiO 2 . In addition, the fluorescence intensity of SiO 2 -Tm(TTA/DBM) 3 phen of different sizes was studied, and it was found that it was the highest when the diameter of SiO 2 -Tm(TTA/DBM) 3 phen was 80 nm. We believe this kind of near-infrared fluorescent hybrid material with a more stable structure and stronger fluorescence will have prospects for food detection, biological detection, and biological imaging.